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2980:Determining a memo's number of lines showing.

KEYWORDS: memo number of lines displayed AREA: General

Q:  How can I tell how many lines a memo is capable of showing?

A:  Here is the short and elegant way that works (most of the
time):

function TForm1.MemoLinesShowing(memo: TMemo): integer;
var
  R: TRect;
begin
  Memo.Perform(EM_GETRECT, 0, Longint(@R));
  Result := (R.Bottom - R.Top) div Canvas.TextHeight('XXX');
end;

The problem with this code is that the TForm and the TMemo must
both be using the same font.  If the fonts are different, then
the calculations are not accurate.

You have to retrieve the font height by selecting it into a
device context. The reason you cannot use the font Height
provided by Delphi is because Delphi caches the font infomation
but doesn't acutally select the font into the DC (canvas) until
it is actually going to draw something. This occurs in the
painting event of the memo.

To get around this problem,  you can get the memo's device
context using the Windows API and  get the font information
from the device context to calculate the text height.

The function below illustrates this process:

function TForm1.MemoLinesShowingLong(Memo: TMemo): integer;
Var
  Oldfont: HFont;  {the old font}
  DC: THandle;     {Device context handle}
  i: integer;      {loop variable}
  Tm: TTextMetric; {text metric structure}
  TheRect: TRect;
begin
  DC := GetDC(Memo.Handle); {Get the memo's device context}
  try
   {Select the memo's font}
    OldFont := SelectObject(DC, Memo.Font.Handle);
    try
      GetTextMetrics(DC, Tm); {Get the text metric info}
      Memo.Perform(EM_GETRECT, 0, longint(@TheRect));
      Result := (TheRect.Bottom - TheRect.Top) div
         (Tm.tmHeight + Tm.tmExternalLeading);
    finally
      SelectObject(DC, Oldfont); {Select the old font}
    end;
  finally
    ReleaseDC(Memo.Handle, DC); {Release the device context}
  end;
end;

        TI

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